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Raúl A. Baragiola University of Virginia, Charlottesville, USA r aul@Virginia.edu

Synergism in magnetosphere-exosphere-ice interactions enhances gas trapping and radiation chemistry. Raúl A. Baragiola University of Virginia, Charlottesville, USA r aul@Virginia.edu. Epistemology. Most of what we observe is the surface Models guide to interpret observations

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Raúl A. Baragiola University of Virginia, Charlottesville, USA r aul@Virginia.edu

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  1. Synergism in magnetosphere-exosphere-ice interactions enhances gas trapping and radiation chemistry Raúl A. Baragiola University of Virginia, Charlottesville, USA raul@Virginia.edu

  2. Epistemology • Most of what we observe is the surface • Models guide to interpret observations • But models are underdetermined by data • Laboratory simulations constraint possibilities • Historically, study each process in isolation, then synthesize full picture

  3. Laboratory Simulations • Standard simulation goals • Exosphere: typ. 10-10 to 10-8 Torr • Temperature: <160 K • Ice: water, water + other gases, rocks • Ice from vapor deposition • Irradiation: particle type, energy, fluxes (?) • Time (not possible) • Gravity: usually ignored

  4. Synergy • Phenomena happen simultaneously • Evaporation, sputtering, photodesorption, condensation, ion implantation, topographical alterations • Previously, each phenomenon studied separately • We started to study 2 at a time

  5. Origin of condensed O2, ozone at Ganymede Condensed O2 Telescope: Spencer et al., J. Geophys. Res. 1995 Ozone Absorption by (O2)2 Lab: Bahr & Baragiola, J. Geophys. Res. 1998 Noll et al., Nature 1996 Vidal, Bahr, Baragiola, Peters, Science 276, 1839 (1997)

  6. H2 O2 H2O Sputtering and generation of atmospheres Escape vs. redeposition Ion

  7. No model accounts quantitatively for condensed oxygen and ozone at Ganymede and some other satellites

  8. Radiation of ice in lab gives H2O2, O2, but no ozone Experiments show Sputtering of O2 (more for heavy ions) O2 trapped in ice (not enough to explain Ganymede) (up to 30% close to surface) H2O2 <1 % No ozone O2 from radiolysis with 100 keV Ar+ Depth profile: Teolis et al, Phys Rev B (2005) ID of H2O2 in Europa, Loeffler & Baragiola Geophys. Res. Lett. (2005)

  9. Water co-deposition enhances oxygen trapping and ozone synthesis Hartley band Teolis, Loeffler, Raut, Fama & Baragiola, Astrophys. J. Letters 644, L141 (2006) Solves the Problem of Ozone on Ganymede (?)

  10. Is exospheric Oxygen trapped in the surface ice?

  11. O2adsorption / desorption cycle in amorophous, porous ice • Ice film grown at 70K, then cooled to 50K. • O2 pressure: of 5.5*10-7 Torr, 90 ML ofO2 are adsorbed. • When removing the O2 ambient the trapped O2 diffuses out

  12. Ion-induced Compaction of Nanoporous Ice OH vibrations in dangling bonds Fluences 10x smaller than for amorphization Surface and volume decay differently with ion fluence Dangling bonds in internal surface Raut, Teolis, Loeffler, Vidal, Famá & Baragiola, J. Chem. Phys. 126 (2007) 244511 • Raut, Famá, Loeffler & Baragiola, Astrophys. J. 687 (2008) 1070ion fluence

  13. Ice in space has been subject to prolonged irradiation, and therefore compacted. Then how can it trap gases (e.g., in comets, icy satellites)?

  14. When O2 is pumped out, the trapped O2 does NOT diffuse out Ion-enhanced adsorption and trapping 2 µm ice film grown at 70K 50 KeV H+ 4 x 1011 /cm2 s When O2 is pumped out, the trapped O2 diffuses out Shi, Teolis & Baragiola, Phys Rev B 79 (2009) 235422

  15. Conclusions • Without irradiation, adsorption above 70K is negligible. The amount of O2 adsorbed depends on film thickness and temperature. Adsorbed O2 cannot be trapped permanently above 50K. • Ice compacted by irradiation in vacuum cannot adsorb gases. • Irradiation enhances gas adsorption and retention at 50K. The enhancement depends on ion flux, ice thickness, ambient pressure as well as the continuity of the ion flux.

  16. UV irradiation under gas exposure Not from closing pores Enhanced O2 absorption with 193 nm light Shi, J. et al. 2011, ApJ Lett. 738, L3 But from radiation chemical products: hydrogen peroxide and ozone

  17. Implications • 193 nm photons can photolyse oxygen and penetrate ~2 meters in the ice, much deeper than ionizing radiation. Thus, they can produce radiation effects deeper in the surface than previously considered. • Deep photolysis does not require nanopores. It could happen in loose grain structure of icy regoliths (macro porosity). Pores significantly increase the residence time of adsorbed molecules, enhancing photodissociation, and favoring molecular synthesis. Shi, J. et al. 2011, ApJ Lett. 738, L3

  18. Astrophysical ices have gas-filled pores stabilized by radiation

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